This invention relates to the thermochemical desurfacing of metal workpieces, commonly called scarfing. More specifically, this invention comprises a method and apparatus for preheating the surface of a metal workpiece where a scarfing reaction is to be started.
A complete scarfing cycle usually consists of three steps: (1) positioning the workpiece in register with the scarfing units, (2) preheating the workpiece to form a molten puddle, and (3) carrying out the scarfing reaction with a stream of scarfing oxygen while causing relative motion between the workpiece and the scarfing unit or units. This invention is concerned principally with the preheating step.
The prior art discloses several methods for performing the preheating step. Jones et al, in U.S. Pat. No. 2,267,405, discloses preheating with a flame produced by combining oxygen and fuel gas within a torch and igniting the gas mixture as it leaves the torch. The problem with combining oxygen and fuel gas within a torch, hereinafter referred to as "premixing", is that the explosive mixture is subject to flashback, i.e. ignition inside the torch, which can damage the torch and become a safety hazard.
An improvement in the pre-mixed flame was disclosed by Jones et al in U.S. Pat. No. 2,356,197, in which oxygen and fuel gas are combined just prior to being discharge from the nozzle. While this was an improvement in the state of the art, the apparatus was still subject to flashback. If the outer nozzle were to be plugged, e.g. with spattered metal, while the oxygen and fuel gas holes inside the unit remained open, the two gases could mix inside the unit, thereby creating an explosive mixture subject to flashback.
Allmang's U.S. Pat. No. 3,231,431 discloses postmixed preheating apparatus wherein the oxygen and fuel gas are combined outside the unit, thereby completely eliminating the possibility of flashback. However, the intensity of the flame produced by this post-mixed apparatus is limited. While Allmang's method can be used to preheat hot workpieces, its low intensity flame requires an unacceptably long time to preheat cold workpieces.
Lytle's U.S. Pat. No. 3,752,460 discloses post-mixed preheating apparatus that uses a stream of "trap" oxygen to decrease preheating time. While Lytle's invention is an improvement over Allmang, Lytle's apparatus is not capable of preheating relatively cold workpieces fast enough for commercial operations.
Engel's U.S. Pat. No. 3,966,503 discloses a method for making an instantaneous scarfing start, that reduces the time required for preheating the workpiece virtually to zero. Engel's method is faster than the method of the present invention; however, Engel's method requires a rod feed mechanism and a high intensity jet of oxygen, not required by the present invention. Hence, the present invention is advantageous when an instantaneous scarfing start is not required, but a fast start on cold steel is desired.
Until the present invention, it has not been possible to rapidly preheat a portion of the surface of a relatively cold metal workpiece to scarfing temperature, using a flame, without danger of flashback, or without using rods, high intensity blowpipes or other adjuvant material.